Only reference I can come up with, pages 11-8 and G-27 of Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B.
Service ceiling. The maximum density altitude where the best rate-of-climb airspeed will produce a 100-feet-per-minute climb at maximum weight while in a clean configuration with maximum continuous power.
The same seems to apply to jets when in a thrust limited condition.
https://www.faa.gov/other_visit/avia...Operations.pdf